1. Field of the Invention
The invention relates to crystals for non-linear optics, to the fabrication of these crystals and to the applications of these crystals.
2. Discussion of the Background
The crystals used in non-linear optics belong to various families, each exhibiting quite specific properties. Historically, one of the first products which appeared in this field was potassium dihydrogen phosphate (KDP). This material is still very widely used because of the relative ease of fabrication, and consequently its relatively low cost. On the other hand, KDP is very sensitive to water, which means that there are several constraints on the way in which it is used. Its second harmonic coefficient is small, which results in relatively low emission of frequency-doubled radiation. Although KDP may easily form good-sized single crystals, which may be required when it is necessary to be able to handle relatively high powers, most crystals for non-linear optics are of small size in practice. This is because they are usually produced by flux-determined growth. This is the case for BBO, LBO and KTP. In this mode, growth is very slow, requiring several weeks or even several months to reach sizes suitable for most uses.
It has been proposed to form crystals by congruent melting, using the Czochralski or Bridgman-Stockbarger techniques. This is the case, for example, for LiNbO.sub.3 crystals. LiNbO.sub.3 crystals exhibit the property of being photorefractive which, for secondharmonic generation, is a drawback. Finally, LiNbO.sub.3 crystals are very brittle. LaBGeO.sub.5 may also be formed by melting. However, it is difficult to obtain because of the appearance of undesirable phases, unless the crystallization operation is perfectly controlled. Moreover, this crystal provides only a relatively low non-linear susceptibility coefficient.